Vehicles, such as aircraft, typically utilise one or more power distribution units to distribute power from a primary power source to various vehicle systems. A power distribution unit typically includes at least one electronic switch, such as a FET, and electronic circuitry that provides wiring protection. The electronic switch and circuitry are usually built in semiconductor technology and therefore referred to as a solid state power controller (“SSPC”). SSPC's have found widespread use because of their desirable status capability, reliability, and packaging density. SSPCs are gaining acceptance as a modern alternative to the combination of conventional electromechanical relays and circuit breakers for commercial aircraft power distribution due to their high reliability, “soft” switching characteristics, fast response time, and ability to facilitate advanced load management and other aircraft functions. FIG. 1 shows a schematic of a standard SSPC application in a power distribution system 10 using only one SSPC 12. The electrical power distribution system 10 distributes power (DC power or AC power) from an electrical power generator 14 to a load circuit 16. SSPC 12 is controlled via a control interface 22.
In aerospace, electrical power distribution SSPCs are used to switch the voltage from the power sources (e.g. generators or batteries) to the loads. Historically, these SSPCs are designed for a given current rating (e.g. 3A, 5A, 10A . . . ). While SSPCs with current rating under 15 A have been widely utilised in aircraft secondary distribution systems, power dissipation, voltage drop, and leakage current associated with solid state power switching devices pose challenges for using SSPCs in high voltage applications of aircraft primary distribution systems with higher current ratings.
An approach to provide more flexibility is to allow the paralleling of SSPCs, where the electronic switches contacts are configured such that the SSPCs share the load current. So the SSPCs can be used stand-alone or in parallel dependent on load requirements. This allows achieving larger current ratings using a plurality of SSPCs having a lower current rating connected in parallel. FIG. 2 shows a schematic of a paralleled SSPC application in a power distribution system 10 using two SSPCs 12a and 12b connected in parallel. The electrical power distribution system 10 distributes power (DC power or AC power) from electrical power generator 14 to load circuit 16. SSPC 12a is controlled via a control interface 22a and SSPC 12b is controlled via a control interface 22b. It be understood that the number of SSPCs connected parallel is not limited to two, but may be any number as desired to achieve a desired current rating.
A typical SSPC generally comprises a power section including at least one solid state switching device (SSSD) which performs the primary power ON/OFF switching, and at least one control section, which is responsible for SSSD ON/OFF control and feeder wire protection. A typical power distribution unit may include hundreds or thousands of SSPCs.
While connecting a number or SSPCs in parallel is a good conceptual approach for flexibility, due to a number of technical reasons implementation has turned to be rather difficult. One problem is that the current sharing between the SSPCs connected in parallel is not perfect. This is due to fact that each SSPC has a slightly different switch resistance (because of manufacturing tolerances). Moreover, the SSPCs are mounted on printed circuit boards (PCB) and it has turned out that PCB resistance may be different between individual PCBs. As consequence, the paralleled SSPCs do not provide the double current rating (e.g. 2×5A=10A in case of two paralled SSPC with a current rating of 5A for each individual SSPC). Rather, the maximum current rating will be lower and depend on the actual current balance achieved.
It is desirable to have an SSPC design which allows overcoming the above problems.